If your after rats then it's going to be low light conditions, a zoom scope will allow you to drop the mag down and brighten the image, but then allow you to increase the mag and shoot farther when it suits you.

Mildot reticle so easy to see in low light and good build quality/lenses for the money - http://www.uttings.co.uk/p112248-bus.../#.WU1ipU2WyUk